This invention relates generally to a step motor driving mechanism in an electronic timepiece, and in particular to a step motor driving control circuit for reducing the current required to drive a step motor by applying drive signals having a pulse width of a duration corresponding to the load placed on the step motor.
The widespread acceptance of electronic wristwatches, having electronic movements and utilizing a quartz crystal vibrator as a time standard is, in large measure, a result of extremely accurate timekeeping operation performed thereby, as well as the reliability offered by such wristwatches. One effort at improving the reliability of such timepieces has been directed to reducing the current consumption thereof, in order to reduce the rate at which the DC battery utilized to energize same is dissipated and thereby reduce the frequency with which the battery needs to be replaced.
Although the average power consumption of electronic wristwatches that were initially developed was on the order of 20 .mu.W, the average power consumption has been reduced to approximately 5 .mu.W. Specifically, in the timekeeping circuitry which includes the oscillator circuit, divider circuit and control circuitry therefor, the average power consumption is 1.5 to 2.0 .mu.W. The remaining power consumption occurs in the electro-mechanical converter of the electronic wristwatch and is on the average of 3 to 3.5 .mu.W. Thus, the average power consumption resulting from the driving of the step motor, or other electro-mechanical converter, accounts for 60% to 70% of the entire power consumption of the electronic timepiece movement.
Although efforts have been made to reduce the power consumption of the electro-mechanical converter, these efforts have met with little success. Specifically, electro-mechanical converters have been developed that have a particularly high degree of efficiency, and hence the reduction in power consumption, if any, that will be gained from increasing the degree of efficiency of the electro-mechanical converter would be substantially insignificant. Moreover, the electro-mechanical converting mechanisms utilized in electronic wristwatches often consume additional power as a result of the inclusion of temperature, calendar and other environmental measurement mechanisms in the wristwatch. Also, an increase in power consumption results from vibration, shocks and other disturbances resulting from the normal use of the wristwatch. Accordingly, the electro-mechanical converting mechanism must be designed to effect driving of the gear train by the rotor under the worst operating conditions that can be anticipated.
For example, when a timepiece includes a calendar mechanism, an additional load is placed on the step motor four or five hours of the day with little, or no, additional load being placed on the step motor the remaining twenty of so hours of the day. In order to accommodate the calendar mechanism in the wristwatch, the electro-mechanical converter mechanism must be designed to drive the motor under the worst conditions, namely, when the calendar mechanism is being operated, thereby resulting in unnecessary power consumption occurring during the remaining twenty or so hours of the day. Accordingly, an electronic wristwatch, wherein the current consumption of the step motor is substantially reduced by varying the pulse width of the drive signal applied thereto in relation to the load condition placed on the step motor, is desired.